Batteries are used in a variety of fields including notebook type personal computers (PC), personal digital assistants (PDAs), digital still cameras (DSC), smart-phones, electric automobiles, and motor-assisted bicycles.
Batteries such as lithium ion batteries for use in notebook PCs are likely to burst if they are overcharged during charging. On the other hand, if they are discharged exceedingly above its allowable limit, their charging/discharging characteristics are deteriorated.
In order to circumvent such incidents, and take advantages of the battery performance, appropriate charging and discharging control suitable for the charging/discharging characteristics is required. For this purpose, there have been implemented in the past measurement devices for monitoring charging and discharging status of batteries.
A typical discrete measurement device measures a charging/discharging current that varies over a wide range (e.g.0.5 mA–15 A), converts it to a voltage (5 μV–150 mV) and holds sampled voltage after it is amplified. This type of measurement device also includes a micro-controller unit (MCU) for converting the voltage into a digital signal by means of a multi-bit (e.g. 10 bits) sequential comparison type A/D converter and for averaging the data obtained.
Since the conventional measurement device integrates the charging/discharging current by accumulating the product of sampled discrete value and the sampling period, the integrated value inevitably has an error. In addition, precise measurement is difficult due to the fact that, since the dynamic range of the input signal is large, the measurement is influenced by a persistent weak current and noise. Further, in order to obtain practical accuracy in the averaging process, it is necessary to repeat a prolonged measurement using an MCU that consumes a fairly large current, which makes it difficult to reduce energy consumption.
FIG. 1 is a block diagram showing a conventional integration type A/D converter for integrating a continuous variable and converting it into a digital variable through A/D conversion. FIG. 2 is a timing diagram of the converter.
As seen in FIGS. 1 and 2, the integration circuit 701 of the converter is initially reset, providing zero Volt as the output Ea thereof (the output of the integration circuit hereinafter referred to as integral output voltage). Under this condition, a start pulse s is applied to a flip-flop 703, which turns on switch S1 and turns off switch S2 to couple the converter to an input signal Ei. Then the integration circuit 701 starts integration of the input signal Ei, generating an output voltage −Ei/RC. If the output Ea exceeds a comparative voltage −ΔVt, a comparative pulse p is generated, starting a first period of integration (the period hereinafter referred to as integration period). The integration period Ts is the time basis of the measurement.
AND circuit 706 is now opened to cause counter 704 to count the number of clock pulses issued from clock generator 705. As the count of the counter 704 reaches an overflow value Nm after a period Ts, an overflow pulse r is generated to reset the flip-flop 703.
Next, the switch S1 is turned off and switch S2 turned on, switching the connection to a reference voltage of −Es, starting the integration of the current in a second integration period T. This results in an output of the integration circuit 701 having an opposite slope Es/RC as compared with the first integration. As the output Ea returns to −ΔVt, the output of comparator 702 is inverted, closing the AND circuit 706. During this period, the counter 704 has been again counting the clocks starting from 1 after the overflow. The count N of the counter 704 at the time the AND circuit 706 is closed is proportional to the input signal Ei.
The value of the integration obtained by the integration circuit 701 in the first half of the integration is proportional to the level of the input signal Ei, while in the second half integration in the reverse direction the period T of integration is proportional to the level of the input signal Ei. That is, Ei·Nm=Es·N. Thus, the input signal Ei (=N·Es/Nm) is A/D converted by counting the clock pulses N during the period T. Such conventional integration type A/D converter has advantages in that it is not costly assembled and not strongly influenced by noise.
The conventional A/D converter requires, in addition to integration periods for integrating input signal, a reverse integration time (i.e. time for A/D conversion) for discharging the integrated charge. Hence, the measurement of an integrated value of a continuous input signal involves time interval in which no A/D conversion is made.
On account of these intervals, it is difficult to obtain accurate integration of the input signal if a compensation is made for the intervals by, for example, averaging the input signal over the intervals. Particularly, in the measurement of charging/discharging current of a battery, this can be a source of a large error, since then the input signal varies over a wide range.
In addition, data varies in conventional analog A/D converter due to offsets and fluctuations in characteristics of the amplifiers and comparators used, and due to a change in temperature.
For these reasons, it has been difficult to improve the accuracy of the measurements.